The invention relates to a metallic heat transfer tube, in particular for the evaporation of liquids from pure substances or mixtures oriented on the outside of the tube.
In the following discussion, certain specific terminology will be used. The phrase xe2x80x9cshellsidexe2x80x9d is to refer to the outside region of a tube. The phrase xe2x80x9ctubesidexe2x80x9d is to refer to the inside region of a tube.
Evaporation occurs in many fields of air-conditioning and refrigeration engineering and process and energy engineering. In engineering often so-called shell and tube heat exchangers are utilized in which liquids from pure substances or mixtures evaporate on the shellside and thereby cool off a brine or water on the tubeside. Such apparatus are identified as flooded evaporators.
By intensifying the heat transfer on the shellside and the tubeside, it is possible to significantly reduce the size of the evaporator. This reduces the manufacturing costs of such apparatus. Furthermore, the necessary filling capacity of refrigerant is reduced, which refrigerant, in view of the present day predominant use of HFCs, adds up to a significant portion of the costs of the entire system. In the case of toxic or combustible refrigerants, it is furthermore possible to reduce the potential of danger by reducing the filling capacity. The double enhanced tubes, which are common today, are more efficient approximately by a factor three then plain or smooth tubes with the same diameter.
The present invention relates to structured tubes, in which the shellside heat transfer coefficient is intensified. Since through this the main portion of the heat transfer resistance is often shifted to the inside of the tube, it is as a rule also necessary to intensify the heat transfer coefficient on the inside. Heat transfer tubes for shell and tube heat exchangers have usually at least one structured area and smooth ends and possibly smooth center lands. The smooth ends or smooth center lands confine the structured areas. In order for the tube to be able to be installed without any problems into the shell and tube heat exchanger, the outside diameter of the structured areas may not be greater than the outside diameter of the smooth ends and smooth center lands.
To increase heat transfer during evaporation, the process of the nucleate boiling is intensified. It is known that the formation of bubbles starts at the nucleation sites. These nucleation sites are mostly small gas or vapor inclusions. Such nucleation sites can be created merely by roughening the surface. When the increasing bubble has reached a specific size, it detaches from the surface. When during the course of the detachment of the bubble the nucleation site is flooded by the following flow of liquid, the gas or vapor inclusion is possibly displaced by liquid. The nucleation site is in this case inactivated. This can be avoided by suitably designing the nucleation sites. It is necessary for this purpose that the opening of the nucleation site is smaller than the cavity therebelow, as for example in re-entrant cavities.
It is state of the art to manufacture such cavities on the basis of integrally finned tubes. Integrally finned tubes are finned tubes in which the fins are formed out of the wall material of a plain or smooth tube. Various methods are known whereby the channels between adjacent fins are closed off in such a manner that connections between channel and surrounding area remain in form of pores or slots. Liquid and vapor can be transported through these pores or slots. Such essentially closed channels are created in particular by bending or folding of the fin (U.S. Pat. No. 3,696,861, U.S. Pat. No. 5,054,548), by splitting and flattening of the fin (DE 2,758,526, U.S. Pat. No. 4,577,381), and by notching and flattening of the fin (U.S. Pat. No. 4,660,630, EP 0,713,072, U.S. Pat. No. 4,216,826).
The known patents have the goal to produce an as much as possible constant channel and pore size. The U.S. Pat. No. 5,054,548 discloses depending on the substance to be evaporated (high pressure or low pressure refrigerant) optimal pore sizes of different sizes. This consideration assumes that the pore system is best constructed of equally large pores.
JP OS 63-172,892 describes a method with which large and small cavities are created that are closed off from one another. This is accomplished by widening the rolled fin channels at regular intervals. The individual cavities are connected to the outside area by variably large pores; however, large and small cavities are separated from one another. The goal of the JP OS 63-172,892 is to create a structure which is supposed to function steadily during variable heat fluxes, expressed by the wall superheat. The large cavities and pores are, during high wall superheat, suppose to assure the heat transfer, whereas the small cavities and channels separated therefrom are suppose to assure heat transfer during low wall superheat. This manner of consideration assumes again that a certain pore size is optimal for a specified operating condition (heat flux, equilibrium conditions, evaporating substance). The widening of the channels is achieved by a gearlike disk which is thicker than the channel width between the fins. With this the fins are pressed further apart to both sides at the widening area. The two adjacent channels are subsequently closed off at this area, thus creating individual cavities separated from one another. A comparatively very large opening is created at the widening area.
JP OS 54-16,766 suggests a heat transfer surface with large and small pore openings, whereby the pores are arranged in such a manner that all large pores are on one side of the tube and all small pores on the other side of the tube. Such a tube is provided for the horizontal installation into a shell and tube heat exchanger. However, the installation must be done in such a manner that the large pores are directed upwardly and the small pores downwardly. The liquid is then sucked in through the small pores and the vapor is ejected upwardly through the large pores. Such an installation in a specified orientation can, however, not be carried out during a large scale production of heat exchangers since the tubes are usually connected to the heat exchanger through a rolling operation, and during this rolling operation the tube rotates about its axis at an uncontrollable angle measurement. Furthermore it must be considered that in the case of this tube design the channels must have a very large volume for fluid hydraulic reasons. This results in disadvantageously high tube weights and in a large layer thickness of the outside structure. The latter results in a small inner cross-sectional surface of the tube and thus in an undesired high pressure drop of the fluid flowing in the tube.
U.S. Pat. No. 5,597,039 (or U.S. Pat No. 5,896,660) describes an evaporator tube with bent fin tips, whereby the fin tips are provided with notches prior to bending. Adjacent notches of one fin have hereby a different shape and/or size so that a system of different pore openings is created. It is thereby viewed as being significant that directly adjacent openings differ in size. Depending on the operating condition, expressed by the heat flux, the type of pores favorable for the operating condition is activated. The many different pores have the purpose of lending the tube good evaporation characteristics over a wide range of operating conditions. However, the respectively not active pores do not contribute to the evaporation process. Rather they reduce the density of the active nucleation sites and can thus even worsen the heat transfer characteristics of the tube.
The basic purpose of the invention is to produce a heat transfer tube of the mentioned type with improved characteristics regarding the heat transfer during evaporation of substances on the shellside. The heat transfer characteristics are adaptable to the properties of the substance to be evaporated and to the operating condition.
The purpose is attained according to the invention by the channels extending circumferentially with an essentially constant cross section between the fins being open outwardly through pores with at least two variable sizes, whereby both the ratio of the pore sizes and also the ratio of the number of small and large pores must meet specific conditions.
The size of one individual pore can be precisely defined and can be detected via a measuring technique. Based on the manufacturing process and caused by tolerances in material and tool, two at random selected pores have practically never the same shape and size. The pore size is subjected to statistical fluctuations. It therefore is advisable to divide the pores corresponding to their size into size classes, whereby the pores are grouped with a finite distribution width around maximums of frequency. Pores of variable sizes in the sense of the invention exist when in the histogram according to FIG. 5 the x-coordinates of adjacent maximums of the frequency distribution differ by at least 50% of the x-coordinate belonging to the smallest pore class.
For the determination of pore size and pore frequency distribution via a measuring technique, for example, a suitable image processing system, consisting of an optical scanning unit and digital data processing unit is utilized. The tube surface is detected through photography and the image is sorted in grey tones. By suitably choosing a grey tone threshold, the image of the tube surface is separated into pore areas and areas of a metallic surface. The pore areas are then geometrically measured and digitally evaluated. FIG. 5 illustrates the frequency distribution of the pore size, which frequency distribution has been determined by means of such a system on an inventive tube sample (compare the numerical example, which is dealt with later on). The pore size is characterized by the area of the pore opening, measured in xcexcm2. One recognizes two maximums in the histogram. The class of the small pores is grouped around the maximum with a pore area As, the class of the large pores is grouped around the maximum with a pore area Al. The values A1 and As can thus be interpreted in each case as the average pore size of the two pore classes. The ratio Ns/Nl (number Ns of the small pores compared to the number Nl of the large pores) is identified with m.
The channels between the fins are according to the invention essentially closed off by material of the upper fin regions, whereby the cavities created in this manner are connected by pores to the surrounding area. These pores are designed such that they can be divided into typically two classes. After a regular, repetitive pattern one or several large pores follow along the channels after each one specific number of small pores. An oriented flow in the channels is created by this structure. Liquid is pulled in through the small pores with the support of the capillary pressure and wets the channel walls, thus creating thin films. The liquid evaporates from the thin films. The vapor accumulates in the center of the channel and escapes at the areas with the least capillary pressure. These are the large pores arranged at specific intervals. The size ratio Al/As and frequency ratio m of the small and large pores are chosen in such a manner that the vapor can escape without too much liquid penetrating into the channels and floods same, which would destroy the very effective thin film evaporation. On the other hand, the vapor pores must be chosen sufficiently large so that the vapor does not accumulate back in the pores. Preferably the size ratio Al/As=1.5 to 4, and more preferably Al/As=2 to 3.
The following ratio between the entire opening area Fs of all small pores and the entire opening area Fl of all large pores is valid:             F      s              F      ❘        :=                              ∑          i                ⁢                  xe2x80x83                ⁢                  A                      s            ,            i                                                ∑          j                ⁢                  xe2x80x83                ⁢                  A                      ❘                          ,              j                                            =                                        A            s                    ·                      N            s                                                A            ❘                    ·                      N            ❘                              =                                    A            s                                A            ❘                          ·        m            
The ratio of the entire opening areas must be adjusted to the properties of the substance which is being used. It must hereby be particularly considered when designing the pore geometry that this ratio should be proportional with respect to the square root of the density ratio of vapor xcfx81v and liquid xcfx81L:             F      s              F      ❘        ~                    ρ        V                    ρ        L            
Thus the pore structure can be adapted to the properties of the substance being used and the operating condition, in particular the pressure level.
Subject matter of the invention also includes a method for the manufacture of the inventive heat transfer tube.
Starting out from the method according to U.S. Pat. No. 5,896,660, the method of the invention is characterized by the notching being created by large and small teeth arranged on the circumference of the notching disk; the notched fin tips are flattened by radial pressure to the level of the notching.
An apparatus for carrying out the method of the invention is characterized by the notching disk having small and large teeth at regular intervals over its circumference, whereby in each case a specific number of small teeth is followed by a large tooth or several large teeth, and whereby the ratio m between the number of small teeth and the number of large teeth is 12:1 to 1:5, more preferably 9:1 to 1:3; and a flattening disk follows the notching disk. (This ratio m is naturally identical with m=Ns/Nl, which is the frequency ratio of small and large pores.)